docs: def lower_extension(filename):
      '''
      This is a helper used by :meth:`Storage.save` to provide lowercase extensions for
      all processed files, to compare with configured extensions in the same
      case.
  
      :param str filename: The filename to ensure has a lowercase extension.
      '''
      if '.' in filename:
          main, ext = os.path.splitext(filename)
          return main + ext.lower()
      # For consistency with os.path.splitext,
      # do not treat a filename without an extension as an extension.
      # That is, do not return filename.lower().
      return filename